This application claims priority from Japanese Patent Application P2006-337188, filed on Dec. 14, 2006. The entire contents of the aforementioned application is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a household fuel cell system using a fuel cell. The fuel cell is provided with not a structure in which hydrogen and oxygen are directly burned, but a structure in which they are allowed to separately electrochemically react in a fuel electrode and an air electrode separated by an electrolyte, so that electricity is generated by taking out electrons to an external circuit.
2. Description of the Related Art
With respect to the fuel cell system, such a system has been proposed in which the fuel cell output is made coincident with the load power (power consumed by a load) so as to prevent a current from reversely flowing into the commercial power supply (refer to, for example, Japanese Patent Application Laid-Open No. 2001-068125). For example, this fuel cell system is provided with a fuel cell power supply used for supplying power of the fuel cell, a power conversion means that converts a fuel cell power to an electric power corresponding to a load so as to output the converted power to the load, and a control means that controls the fuel cell power supply and the power conversion means. The fuel cell power supply is provided with a reforming device that reforms a city gas into a hydrogen-rich fuel gas, and a fuel cell that generates electrical energy by using the supplied fuel gas. The power conversion means is constituted by, for example, a DC/DC converter that voltage-raises the fuel cell power and a power conditioner having an inverter used for converting the output of the DC/DC converter to an ac current. The control means is designed to control the fuel cell power supply and the power conversion means depending on the power consumption of the load.
This fuel cell system does not permit the fuel cell power to exceed a specified value and reversely flow into the commercial power supply, and in addition, the fuel cell power is not operative to follow a drastic variation in the load. For this reason, when the load is varied to a low level, an excessive power is generated due to a slow follow-up operation in following the variation. In order to prevent this excessive power from reversely flowing into the commercial power supply, such techniques have been proposed in which the excessive power is thermally converted by a heater to be consumed and a forward current is given from the commercial power-supply side to prevent the reversed current.
In this fuel cell system, however, because the rate of fuel cell power utilization is low due to a slow follow-up property to a drastic variation in the load, the installation rate of this system for practical use is low. Therefore, the applicant of the present invention has made extensive studies to improve the disadvantages of the conventional fuel cell system, and has consequently proposed a novel fuel cell system that can enhance the rate of fuel cell power utilization by installing therein a charging/discharging device, which, completely different from the conventional process for excessive power, allows the excessive fuel cell power to be charged in a capacitor, and upon insufficiency in the cell power, also allows the capacitor to discharge the accumulated power to compensate for the insufficient power.
In this newly developed fuel cell system, high response and high follow-up property to power variations of the capacitor and a capacitor such as a recently-developed electric double layer capacitor having a large capacity as high as a lead storage battery are utilized. For example, Japanese Patent Application Laid-Open No. 2003-199332 has disclosed such an energy storage device that utilizes the electric double layer capacitor.
In the fuel cell system in which the charging/discharging device utilizing a capacitor is assembled however, a problem arises in that, when a capacitor is over-charged to a level exceeding the capacitor withstand voltage (the upper limit value of capacitor voltage) or when a capacitor is over-discharged down to a capacitor discharge depth (the lower limit value of capacitor voltage) or less, the charging/discharging means might be damaged to deteriorate the capacitor.
Therefore, the fuel cell system of the present invention controls so that upon charging, the capacitor is allowed to have a space capacity so as not to exceed the capacitor withstand voltage and, upon discharging, the capacitor is prevented from discharging down to a voltage of the capacitor discharge depth or less. Thus, the fuel cell system enables to prevent the capacitor from deteriorating as well as positively preventing a reversed current from flowing into the commercial power supply, and also to improve the capacity operating rate of the fuel cell.